Contrasting strategies, shared risk: 12-yr nitrogen addition weakens hydraulic safety in young and mature trees via distinct pathways

Nitrogen (N) deposition is widely assumed to stimulate growth in N-limited temperate forests, yet how N deposition interacts with tree ontogeny to regulate the carbon-water processes shaping tree resilience, decline, and mortality under environmental stress remains unclear. We used a 12-year anthropogenic N addition (control; N20: 20 kg N ha-1 year-1; and N50: 50 kg N ha-1 year-1) experiment spanning young, intermediate, and mature larch plantations in northern China to test age-dependent effects on hydraulics and nonstructural carbohydrates (NSCs) reserves. In young trees, N50 decreased soluble sugars in leaves, twigs, and branches, but increased in roots, suggesting a preferential belowground allocation, whereas N20 responses were limited. However, this apparent adaptive response was accompanied by increased xylem embolism vulnerability, indicating a potential trade-off between greater root carbon investment and hydraulic safety. In contrast, mature trees exhibited a tendency of systemic impairment of hydraulic function but with no signs of NSC reallocation. Our results demonstrate that decadal N addition disrupts carbon-water balance age-dependently: young trees trade hydraulic safety for carbon reallocation while mature trees undergo hydraulic decline. These findings challenge the view that N deposition uniformly benefits temperate forests and show that incorporating age-specific physiology is essential for predicting temperate forest dynamics.